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TP01

TP01 is the leading sensor for long-term monitoring of soil thermal conductivity. A measurement with TP01 may also be used to estimate soil thermal diffusivity and volumic heat capacity, leading to a better understanding of dynamic (variable heat flux) thermal behaviour of soils. TP01 is designed for long-term use at one measurement location. Applied in meteorological surface flux measurement systems, TP01 improves the estimates of heat transport in the soil and of the so-called storage term.

Introduction

TP01 measures soil thermal conductivity. It is designed for long-term on-site operation, buried in the soil. Its rated operating range is 0.3 to 4 W/(m∙K), which covers most inorganic soil types. The sensor inside TP01 is a temperature-difference sensor consisting of 2 thermopiles. It measures the radial temperature difference around a heating wire with a record breaking sensitivity. Both the heating wire and the sensor are incorporated in a very thin plastic foil. It can be connected directly to commonly used data logging systems. The low thermal mass of TP01 also makes it suitable for measuring the thermal diffusivity. TP01 should be incorporated in the user's measurement and control system. Typically every 6 hours, the TP01 heater is switched on to perform a measurement. The thermal conductivity, λ, is calculated by dividing the TP01 sensitivity, S, by the sensor output, a small voltage difference ΔU which is a response to stepwise heating, and multiplying by the applied electrical power Q per meter heating wire.The measurement function of TP01 is:

λ = S· Q/ U

The factory-determined sensitivity S, as obtained under calibration reference conditions, is provided with TP01 on its product certificate. Thermal diffusivity and volumic heat capacity are estimated from the time response to stepwise heating. These measurements are optional. The volumic heat capacity is a linear function of soil water content and TP01 may be employed to monitor trends in soil water content. Contrary to many other soil water content sensors, TP01 is not sensitive to contamination by salts and the measurement still functions in electrically conducting saline or fertilised soils.

Suggested use

A typical TP01 is part of a meteorological surface flux measurement system in which also wind, humidity, soil heat flux, soil temperatures at different depths and net radiation are measured. TP01 then serves to improve the estimate of the so-called storage term, which is used to model thermal heat transport in the soil. Measurements with TP01 are often combined with soil temperature profile measurements with sensor model STP01 and measurements with heat flux sensor model HFP01SC. Soil thermal properties change as a function of depth, in particular close to the soil surface. A typical measurement location is equipped with sensors at several depths. For good spatial averaging at least 2 sensors (> 5 m apart) should be installed at every depth. Hukseflux has equipped several testbeds in the electrical power industry, to monitor dryout, thermal runaway and thermal stability around mockup high-voltage power lines. Here the capability to perform a crude measurement of thermal diffusivity is an important feature, for modelling behavior under dynamic loads.